What is the ideal headphone frequency response. Well no one really has a perfect answer to that. One thing we do know is that a “perfectly flat” response – a well established goal for loudspeakers – is not suitable for headphones and earphones.
The problem is firstly made difficult by various ways to measure headphone and earphone responses. In other words, it depends on the test jig.
There are some characteristics about the outer ear that mean we get a different experience when the sound arrives from the free field compared to being delivered in a closed earphone cavity, despite the two having a very similar response shape, perhaps some subtle cues coming from reflection around the pinna or an inconsistency with the vibrations ariving through the body. “the missing 6dB” is a phrase used sometimes to highlight the fact that headphones generally need a strong bass response to sound good, perhaps to compensate for the lack of vibration coming through the body as would be the case for sound from a loudspeaker. The answer really is to sit down with a good parametric EQ and fiddle with it until you get the sound you want. Then that response can be implemented into the headphone.